Supply Chain (solidity)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;
contract supply {
    struct product {
        uint id;
        string name;
        uint manufacturingdate;
        address manufacturer;
        address distributor;
        address retailer;
        string status;
    }
    mapping(uint => product) public products;
    uint public productcount;

    event manufactured(uint id, string name, address indexed manufacturer, uint manufacturingdate);
    event distributed(uint id, address indexed distributor);
    event delivered(uint id, address indexed retailer);
    
    modifier onlymanufacturer(uint _id) {
        require(products[_id].manufacturer == msg.sender, "Only the manufacturer can perform this action");
        _;
    }
    modifier onlydistributor(uint _id) {
        require(products[_id].distributor == msg.sender, "only distributor can perform this task");
        _;
    }
    function addproduct(string memory _name) public {
        productcount++;
        products[productcount] = product({
            id: productcount,
            name: _name,
            manufacturingdate: block.timestamp,
            manufacturer: msg.sender,
            distributor: address(0),
            retailer: address(0),
            status: "Product added"
        });
        emit manufactured(productcount, _name, msg.sender, block.timestamp);
    }
    function shipproduct(uint _id, address _distributor) public onlymanufacturer(_id) {
        products[_id].distributor = _distributor;
        products[_id].status = "Shipped";
        emit distributed(_id, _distributor);
    }
    function delivertoretailer(uint _id, address _retailer) public onlydistributor(_id) {
        products[_id].retailer = _retailer;
        products[_id].status = "Delivered";
        emit delivered(_id, _retailer);
    }
    
    function getproductdetail(uint _id) public view returns (uint, string memory, address, string memory) {
        product memory p = products[_id];
        return (p.id, p.name, p.manufacturer, p.status);
    }
}

Here’s a line-by-line explanation of your Solidity smart contract named supply, which models a simple supply chain for tracking products from manufacturer → distributor → retailer.

---

✅ File Info

// SPDX-License-Identifier: MIT

• License declaration: Makes the contract open-source under the MIT License.

pragma solidity ^0.8.20;

• Specifies the Solidity compiler version (at least 0.8.20).

---

🏗 Contract Declaration

contract supply {

• Declares the contract named supply.

---

📦 Struct Definition: product

    struct product {
        uint id;
        string name;
        uint manufacturingdate;
        address manufacturer;
        address distributor;
        address retailer;
        string status;
    }

Defines a product with the following fields:

• id: Unique ID for each product
• name: Product name
• manufacturingdate: Timestamp of creation
• manufacturer: Address that created the product
• distributor: Address to which the product is shipped
• retailer: Address of the final delivery
• status: Current status (e.g., “Product added”, “Shipped”, “Delivered”)

---

🗺 State Variables

    mapping(uint => product) public products;

• Stores all product structs by their id.

    uint public productcount;

• Counter to assign unique product IDs.

---

📣 Events

These are used for logging actions on-chain, visible to frontend apps:

    event manufactured(uint id, string name, address indexed manufacturer, uint manufacturingdate);
    event distributed(uint id, address indexed distributor);
    event delivered(uint id, address indexed retailer);

• manufactured: Emitted when a product is created.
• distributed: Emitted when shipped to a distributor.
• delivered: Emitted when delivered to a retailer.

---

🛡️ Modifiers

Modifiers are reusable access-control rules.

👨‍🏭 Only Manufacturer

    modifier onlymanufacturer(uint _id) {
        require(products[_id].manufacturer == msg.sender, "Only the manufacturer can perform this action");
        _;
    }

• Ensures that only the product’s manufacturer can perform the action.

🛒 Only Distributor

    modifier onlydistributor(uint _id) {
        require(products[_id].distributor == msg.sender, "only distributor can perform this task");
        _;
    }

• Ensures that only the assigned distributor can perform the next step.

---

🛠️ Core Functions

1️⃣ Add Product (Create)

    function addproduct(string memory _name) public {
        productcount++;

• Increments product ID counter.

        products[productcount] = product({
            id: productcount,
            name: _name,
            manufacturingdate: block.timestamp,
            manufacturer: msg.sender,
            distributor: address(0),
            retailer: address(0),
            status: "Product added"
        });

• Creates a new product:
  • msg.sender is set as the manufacturer.
  • distributor and retailer are initialized as address(0) (i.e., unset).
  • Status is set to "Product added".

        emit manufactured(productcount, _name, msg.sender, block.timestamp);
    }

• Emits manufactured event.

---

2️⃣ Ship Product to Distributor

    function shipproduct(uint _id, address _distributor) public onlymanufacturer(_id) {

• Only the manufacturer of the product can call this.

        products[_id].distributor = _distributor;
        products[_id].status = "Shipped";
        emit distributed(_id, _distributor);
    }

• Assigns the distributor address.
• Updates the status.
• Emits distributed event.

---

3️⃣ Deliver Product to Retailer

    function delivertoretailer(uint _id, address _retailer) public onlydistributor(_id) {

• Only the assigned distributor can call this.

        products[_id].retailer = _retailer;
        products[_id].status = "Delivered";
        emit delivered(_id, _retailer);
    }

• Assigns the retailer.
• Updates status.
• Emits delivered event.

---

4️⃣ View Product Info

    function getproductdetail(uint _id) public view returns (uint, string memory, address, string memory) {
        product memory p = products[_id];
        return (p.id, p.name, p.manufacturer, p.status);
    }

• Allows anyone to view key product details: id, name, manufacturer, and current status.

---

✅ Summary Table

Feature	Description
addproduct()	Manufacturer creates a new product
shipproduct()	Ships product from manufacturer to distributor
delivertoretailer()	Distributor delivers product to retailer
getproductdetail()	View basic info about the product
onlymanufacturer	Access control for manufacturer-only actions
onlydistributor	Access control for distributor-only actions
status	Tracks progress: "Product added" → "Shipped" → "Delivered"
events	Used for tracking and integration with frontends/dApps